The present invention relates to a device for measuring the flow rate of a gas through a conduit, and in particular for measuring the flow rate of breathable gas flow associated with a non-invasive positive pressure ventilation (NIPPV) apparatus, continuous positive airway pressure (CPAP) apparatus, ventilatory assist device, or the like.
The term xe2x80x98conduitxe2x80x99, used herein, is to be understood in a non-limiting sense. Particularly, it is not limited only to a flexible air hose that interconnects a flow generator with a patient mask for CPAP or assisted ventilation equipment.
In this specification, the phrases xe2x80x98encodexe2x80x99, encoderxe2x80x99 or encodedxe2x80x99 are to be understood as measuring any apparatus or operation that effects the fraction of transmitted light which is received by a detector.
NIPPV apparatus function to supply a patient with clean breathable gas (usually air, with or without supplemental oxygen) at a therapeutic pressure or pressures, at appropriate times during the subject""s breathing cycle. Such apparatus includes a blower to produce a flow of pressurised breathable gas, a gas supply conduit to receive said gas and an interface, typically a nasal mask, in turn to deliver the gas to the patient""s airways. Further, there is a controller having control over operation of the blower, and a flow measuring device. Typical pressures delivered are in the range of 2 to 30 cm H2O. Typical flow rates are in the range of xe2x88x92150 to 300 L/min. For certain applications in the field of NIPPV and CPAP it is necessary to detect flow signals with a frequency in the range 0 to 30 Hz.
Measurement of the volumetric flowrate of gas (xe2x80x9cflowxe2x80x9d) in CPAP or ventilatory assistance devices is required for calculating the subject""s ventilation, and to detect changes between the inspiratory and expiratory phases of breathing. It is very important that flow be determined accurately.
Typically, fixed or variable orifice meters are used to determine flow. In thes meters, a pressure transducer measures the pressure differential across a fixed or variable orifice. Problems with orifice meters include high pressure drop, poor gain and poor linearity at low flows.
Another known device for measuring the flow rate of a gas is the moving vane flow meter. In these meters, movement of the vane varies or produces changes in an inductive or capacitive electrical signal. Problems with the moving vane transducers include poor dynamic response and limited bandwidth. A further disadvantage of the moving vane transducer is that variations in ambient air temperature and air density can result in measurement errors. This is because the vane transducer inherently measures air mess, rather than air volume.
More sophisticated devices such as thermal flow meters (which measure the molecular flow of gas) and turbines type devices are generally too large and expensive for applications in the field of CPAP and NIPPV.
It is an object of the invention to overcome or at least ameliorate one or more the disadvantages of the prior art.
The invention discloses a measuring device for determining gas flow through a conduit, said device comprising:
a vane arrangement extending into the conduit to be displaceable under the influence of gas flow in the conduit;
a light source arranged to provide a uniform intensity of light over a range of displacement of the vane arrangement;
an optical encoder interposed between the light source and the vane arrangement effective over the range of displacement to encode the light source as a function of displacement of the vane arrangement under the influence of gas flow; and
a light detector arranged to optically communicate with the encoded light source and provide an output signal based thereupon related to gas flow.
In one form, the vane arrangement forms at least one light guide, and the encoder is interposed between one or more free ends of the light guide and the light source so that the encoded light is optically communicated to one or more light guides. In another form, the vane arrangement has two light guides, and the encoder has two rows of regularly spaced discrete light sources aligned with a free end of the respective light guide. In a further form, the vane arrangement carries a first polarizing element, and the first polarizing element and a second, fixed polarizing element form the optical encoder. The light source, first and second polarizing elements and the light detector are in optical alignment, with the planes of polarization of the respective first and second polarizing elements interacting as a function of displacement of the vane element to result in a range of optical transmitivities.
A measuring device as described above can be included in CPAP or ventilatory assistance devices. Such apparatus includes a blower to produce pressurized breathable gas, a gas supply conduit to receive said gas and a device, typically a mask, in turn to deliver the gas to the patient""s airways. Further, there is a controller having control over operation of the blower, and the flow measuring device provides its output gas flow signal to the controller as a control variable therefor.
The invention further discloses a method for detecting gas flow through a conduit, comprising the steps of:
encoding the light source as a function of a displacement of a vane arrangement located within the conduit under the influence of gas flow; and
detecting the encoded light and producing an output signal representative thereof related to gas flow.